Added CurrentAccountId to IProvider

[shweaver-MSFT]

Fixes #

PR Type

What kind of change does this PR introduce?

• Feature

What is the new behavior?

This PR introduces a new property on IProvider called CurrentAccountId, which reflects the identifier of the currently signed in user account.

• For MsalProvider the id comes from Microsoft.Identity.Client.AccountId.Identifier
• For WindowsProvider the id comes from Windows.Security.Credentials.WebAccount.Id
• For MockProvider the id is a dummy value, "mock-account-id".

When NOT signed in CurrentAccountId returns null.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

Other information

[ghost]

Thanks shweaver-MSFT for opening a Pull Request! The reviewers will test the PR and highlight if there is any merge conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌

[shweaver-MSFT]

This comes from an internal request.

Also, don't forget to merge this into main as well.